Prevention of parasitics in electron emission tube circuits



May 12, 1931. I ,c. w. HANSELL 1,804,830

PREVENTION OF PARASITICS IN ELECTRON EMISSION TUBE CIRCUITS Filed Dec. 24, 1928 vwemtoz CLARENCE w. HANSELL 50 parasitics. .I

Patented May 12, 193i UNITED STATES PATENT "OFFICE? CLARENCE w. HANSELL, or PORT JEFFERSON, NEW YORK, ASSIGNOR T RADIO 'ooR- POR-ATION or AMERICA, a CORPORATION or DELAWARE PREVENTION 0F PARASITICS INELEGTRON EMISSION TUBE CIRCUITS Application filed December 24, 1928; eria1-No. 328,135.

This invention relates to the prevention of parasitics in electron emission tube circuits, and more particularly to the prevention in a pushpull electron emissiontube stage of para- ,5 siticsin which the tubes act in parallel or .pushpush.

Pushpull electron emission tube. stages may be neutralized against parasitics of the fundamental frequency by neutralizing condensers connected between the anodes of each of the tubes and the control electrodes of the other of the tubes. This, however, does not prevent parasitics flowing through both halves of the input circuit in parallel, through the tubes in parallel, then through both halves of the output circuit in parallel, and thence back to the input circuit. To overcome this type of parasitic by using resistors in series with the control electrodes of the tubes may be satisfactory withtubes of very small power, such asare employed in receivers, but is not desirable for high powered transmitter stages, and to provide an improved method and means to prevent such parasitics Without appreciable energy loss is the primary object of my invention. To this end I decrease the potential fluctuation on the control electrodes of the tubes, considered in parallel, relative to that onthe anodes of u so the tubes, below the pointof sustaining self oscillation. This I do by means of a loading impedance inserted in the common anode to cathode circuit of the stage, preferably outside of the resonant output circuit. The impedance may best be connected at one terminal directly, to the mid point of the inductance of the tuned output circuit,and at the other terminal directly to a blocking condenser and thence to. ground, assuming the tube cathodes to be grounded. Greater relative anode to cathode impedance is the same as smaller relative control electrode, to cathode impedance, and so causes afsm'aller parasitic potential between the control elec- 46 trodes and the cathodes of the tubes. From another viewpoint we may say that the loading impedance detunes the anode, to cathode circuits, relative to the control electrode to cathode circuits, with respect to: pushpush As a further step against the development of the undesired parallel or. pushpush parasitics I may cause them to take the capacitive, rather than the inductive path, between the control electrodes and the oathodes of the tubes, in order to make the parasitic potential applied to ,theicontrol electrodes degenerative in phase. This is most simply accomplished, structurally,,by substituting for the, singletuning condenser of the input circuit a pair of equal capacitances connected in series, and by grounding the point between the condensers. Insuch', case,

in order to prevent the pushpull resonant input circuit from splitting up into two tuned circuits I connect an additional impedance between the mid point of the input inductance and ground, which additionally .insures that the parasitics will take the capacitive path, and which meanwhile does not in- 7 crease the potential'of the parasitics on the control electrodes by increasing the control electrode to cathode impedance because the parasitics take the capacitive path.

Myinvention isdescribed more in detail 7 in the following specification, which is accompanied by a drawing the single figure of which is awiring diagram for a preferred form of my invention.

Referring to thedrawing, there are a pair of electron emission tubes 2, 4, between the control electrodes of which there is coupled a resonant input circuit 6, and between the anodes of which there is coupled a'resonant output circuit 8: These circuits are connected between likeelectrodes of the tubes in series in order to obtain a pushpull operation. A source of direct anode potential10 is connect-ed, preferably through a radio frequency choke 12, to the mid point of. the anode inductance 14. In a somewhat similar manner a source of bias potential is connected to a lead 16,,and, preferablythrough a radio frequency choke 18, to the mid point of the input inductance 20. Blocking condensers 22 and 24c are respectively arranged in parallel with the anode and bias potential sources. Neutralizing condenser 26 and 28 are. cOnnected as shown, from each of the anodes tothe control electrodes of the other tubes, in order to neutralize the stage with respect to pushpull parasitics.

In spite of the neutralizing condensers parasitics may be generated in the tubes acting in parallel. Such parasitics flow from ground through the upper and lower halves of the input circuit in parallel, through the tubes in parallel, and thence back to ground through the upper and lower halves of the output circuit in parallel, the input and'out pedances adjacent the anodes because the regular pushpull currents would then be impeded, and losses would result. The impedances therefore are preferably connected beyond the resonant output circuit on the way to ground, and is exemplified in the drawing by the common'impedance 30. This impedance may be resistive, as well as inductive, and should preferably have one of its terminals connected directly to the mid point of the inductance 1. and the other of the terminals connected directly to the blocking condenser 22 and thence to ground, if it be supposed that the tube cathodes are grounded, as is usually the case. If not, t ie condenser 22 is connected to the cathodes. The impedance 30 is preferably connected with short leads, as shown, because when dealing with these high frequencies the capacitance of a lead of appreciable length is sufiicientlygreat to form a path to ground, and so permit the growth of the parasitics which are 'to'be prevented.

Furthermore, the short direct connections to ground, as indicated, obviate any possible tendency for radio frequency currents to flow in the lead to the anode current source 10, and effectually prevent 'seriesresonance between the capacitance of the choke 12 and the impedance 30.

It should be kept clearly in mind that the impedance 30 is not a true radio frequency choke, but is merely a detuning impedance, and far smaller in magnitude than a choke. Forexample, at a wave length of 33 meters,

in a stage employing 20, kilowatt tubes, the

impedance may be a coil 1 inches in diameter, 6 inches long, containing about 15 turns.

The magnitude is not at all critical, and may I be varied through a considerable range.

As a further precaution against parasitics I may arrange the circuit to insure that'the parasitic currents flow through the capacitive is operatechand therefore tend to take the .capacitive path.

Grounding the capacitance mid point has the disadvantage of making it possible for the input circuit 6 to split up intoan upper and lower resonant circuit each of which might not be exactly tuned to the input frequency, in actual, practice.- To prevent this I- add the impedance 36, which is connected to L:

the mid point of the inductance 20, as shown. This also further insures that'the parasitics will take the capacitive path through the condensers 32 and 34, rather than the inductive path through the blocking condenser 24 the inductance 36, and the halves of the inductance in parallel.

It should be understood that either of the changes in the output circuit and in the input circuit may be employed separately, as

well as both together. Furthermore, the impedances and 36 need not be connected in circuit exactly as shown, it being possible to use separate impedances adjacent the anodes or a single impedance between the point of connection of the choke 12 and the coil 14,

and the condenser 22, or between the condenser 22 and ground, but for very high frequencies I believe the connection illustrated is exceedingly desirable, in order to prevent A the formation of a. path for the parasitics through the capacitance of the leads employed. It is not essential that the stage be operated with the cathodes at ground potential, for any other point may be grounded,

and suitable direct connections substituted for the connections now shown to ground. The radio frequency chokes 12' and 18 are conventional, and may be employed as an extra precaution to keep radio; frequency currents out of the direct current sources, 'butthey are'not essential to the practice of my invention. H Iclaim: I

" 1. The method of preventing pushpush parasitics in a pushpull electron emission tube stage which includes causing the pushpush parasitics to take the capacitive rather than the indu'ctive'path from the control electrodes to the cathodes of the tubes, in order to make the potential of these parasitics applied to the control electrodes degenerative in phase. 2. A pushpull sta e comprising a pair of electron emission tu s having a rounded cathodes, an input circuit couple to the control electrodes of the tubes, a pushpull output circuit comprising parallel connected inductance and capacitance connected be tween the anodes of the tubes, an impedance one terminal of which is connected directly to the mid point of the anode inductance, a blocking condenser connected between the other terminal of the impedance and ground, and a source of direct anode potential con nected through a radio frequency choke to the common terminal of the impedance and the blocking condenser, said impedance being of sufiicient magnitude to prevent parasitic pushpull oscillations.

3. A pushpull stage comprising a pair of electron emission tubes, an input circuit cou pled to the control electrodes of the tubes, a pushpull output circuit comprising parallel connected inductance and capacitance connected between the anodes of the tubes, a direct current circuit connected to the mid point of the anode inductance and including a source of direct anode potential, a radio frequency by-pass circuit connected from the mid point of the anode inductance to the cathodes of the tubes through a blocking condenser, neutralizing condensers connected between the anodes of each of the tubes and the control electrodes of the other of the tubes to prevent pushpull parasitics, and an impedance connected in the radio frequency bypass circuit of sufficient magnitude to prevent pushpush parasitics.

4. A pushpull stage comprising a pair of electron emission tubes having grounded cathodes, an output circuit connected between the anodes of the tubes, a pushpull input circuit comprising a pair of equal capacitances connected in series and an inductance con nected in. parallel therewith, said parallel circuit being connected between the control electrodes of the tubes, a connection between the condensers and ground in order to make the control electrode to cathode circuits of the tubes capacitive with respect to pushpush parasitics, an impedance one terminal of which is connected directly to the mid point of the input inductance, a blocking condenser connected between theother terminal of the impedance and ground, and a source of direct bias potential connected to the common terminal of the impedance and the blocking condenser, said impedance being of sufficient magnitude to prevent the input circuit from dividing itself into two resonant circuits.

5. A pushpull stage comprising a pair of electron emission tubes having grounded cathodes, a pushpull output circuit comprising parallel connected inductance and capacitance connected between the anodes of the tubes, an impedance one terminal of which is connected directly to the mid point of the anode inductance, a blocking condenser connected between the other terminal of the impedance and ground, a source of direct anode potential connected to the common terminal of the impedance and the blocking condenser, an input circuit comprising a pair of equal capacitances connected in se ries and an inductance connected in parallel therewith, said input circuit being connected between the control electrodes of the tubes, 7

of the impedance and blocking condenser,

said anode impedance being of suflicient magnitude to prevent pushpush parasitics, and said input circuit impedance being of suiiicient magnitude to prevent the input circuit from dividing into two resonant circuits.

CLARENCE W. HANSELL.

CERTIFICATE OF CORRECTION.

Patent No. 1,8il4,830. Granted May 12, 1931, to

CLARENGE'W. HANSELL.

it is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3, line 14, claim 2, for "pushpull" read pushpush; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 5th day of April, A. D. 1932.

M. J. Moore, (Seal) Acting Commissioner-of Patents. 

